[Catalist] Possible effect of Saliva when cutting Glass Tubing?

[Leon Harris]

Hi Mike.
I don't think that you can view it that simply.
To cut glass, you need to initiate a crack. I think the saliva/water 
lessens the energy needed to start that process off.
Once you have a crack, the pressure that you supply causes the ionic 
materials in the glass (all the carbonates, borates etc) to mis-align 
and break by coulombic repulsion. (Pure SiO2 is a pig to work with, you 
need a special hydrogen flame that is ferally high temperature 
(technical term that)).
So as long as you have enough play in the crack to be able to move it 
enough to make the +ve ions in the lattice shift enough so that they can 
be pushed against each other, you can break the glass. It is better to 
think of it as a catalyst rather than a source of energy.

What I love about the whole process is the way it confronts some 
conceptions about glass. 1) that it is an ionic mixture. (remember that 
lovely old physics demo where you put a wire around either end of a rod 
an heat it until it passes enough current to light a light bulb. Go 
check out Basim Shakashiri's books, vol 2 I think from memory, but I may 
be wrong).

2) we never deal properly with those network covalent substances. We 
teach that they go on and on and on (like me now but more!). Our sensory 
evidence tells us they are finite. So what happens at the edges? Lots 
and lots of diamonds terminate in -H and -OH. Every time there is an 
earthquake, there are whole ecosystems of slowly growing Archaea that 
feast on the H2 released when the Si-O-Si bonds cleave and then 
hydrolyse water.


Others later, if time permits.

Cheers,
Leon
On 2/05/2016 9:49 AM, Michael McGarry wrote:
> Greetings Leon,
>
> Thanks for your prompt reply to my 02052016 post to CATALIST.
>
> From your supplied URL:
>
> "The amount of energy required to cleave the silicon-oxygen bond 
> decreases by a factor of twenty in the presence of water. This 
> indicates that the water molecule fits into the crack tip and converts 
> a silicon-oxygen bond into two silanol (SiOH) groups."
>
> "Small diameter rod and tubing can be broken easily by hand. The glass 
> is first scored with a file. If at all possible, the scratch should be 
> made with one stroke of the file because sawing with the file tends to 
> widen the scratch and lowers the chances for an even break. The glass 
> should be supported by the bench top because considerable pressure 
> (*F/A*) (about 3 to 6 pounds [*force*]) is needed to make the scratch. 
> *Saliva or water* is placed on the scratch, which will lower the 
> strength of the glass by about 20%. The rod or tubing is then grasped 
> firmly with the scratch between and opposite the thumbs. *The glass is 
> bent at the same time it is pulled apart *and a clean break should 
> result."
>
> Do we agree that 80 % of the energy needed to snap the scratched glass 
> tubing is supplied by the mechanical stress of simultaneously bending 
> and pulling apart the glass tubing at the scratch, with 20% of the 
> energy supplied by the cleaving of the silicon-oxygen bonds along 
> the scratch in the glass by the polar water molecule in the saliva. I 
> am uncertain of the role played by OH - ions in the saliva. Further 
> research is required. Most importantly we know from experience that 
> one must have the correct proportion of bending and pulling to produce 
> a ‘clean’ break in the scratched glass tubing.
>
> Thanks and Best Regards,
>
> Mike McGarry
>
>
>> On 2 May 2016, at 6:34 AM, Leon Harris <leon at quoll.com 
>> <mailto:leon at quoll.com>> wrote:
>>
>> "Saliva or water is placed on the scratch, which will lower the 
>> strength of the glass by about 20%."
>> say 
>> https://chemistry.boisestate.edu/richardbanks/glassblowing/glassblowing_history.htm
>>
>> So the optional bit probably relates to the argument "is 20% worth 
>> the bother"?
>>
>> But yes, hydroxide in saliva, little that it is, is supposed to 
>> enhance the break.
>>
>> Also note that there is non-negligible OH- in neutral water.
>>
>> In my experience of breaking tube, it seems to work best if you get 
>> it within a few moments of scoring: after that the benefit falls off.
>>
>> More on this later, when time permits....
>>
>> Cheers,
>> Leon
>>
>

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